Accurate measurement of liquids at low flow rates has always presented substantial problems. If such liquids are laden with solids which have a propensity of settling out, accurate metering of such liquids becomes more of a problem.
Metering devices often use cavities or chambers in revolving rotors, however, such measuring cavities or chambers often provide dead spots into which suspended solids will settle from the liquid carrier. Over a period of time, the settled solids will change the usable volume of such measuring cavities, and of course, metering errors will occur, and preset calibrations become inaccurate. Typical of such prior metering devices, and their inherent problems as relates to handling liquids laden with solids in suspension, are Helm U.S. Pat. No. 3,195,776; Lloyd U.S. Pat. No. 3,251,511; Calvert U.S. Pat. No. 3,779,712; and Diez U.S. Pat. No. 3,995,777.